Geodynamics. Heat conduction and production Lecture Heat production. Lecturer: David Whipp

Transcription

1 Geodynamics Heat conduction and production Lecture Heat production Lecturer: David Whipp david.whipp@helsinki.fi Geodynamics 1

2 Goals of this lecture Discuss radiogenic heat production in the Earth, how it varies and the implications for geodynamic processes 2

3 Radiogenic heat production Radiogenic heat production, A or H, results from the decay of radioactive elements in the Earth, mainly 238 U, 235 U, 232 Th and K. A is generally used for volumetric heat production and H for heat production by mass. These elements occur in the mantle, but are concentrated in the crust, where radiogenic heating can be significant The surface heat flow in continental regions is ~65 mw m -2 and ~37 mw m -2 is from radiogenic heat production (57%) Concentration Rock Type U (ppm) Th (ppm) K (%) Reference undepleted (fertile) mantle Depleted peridotites Tholeiitic basalt Granite Shale Average continental crust Chondritic meteorites Turcotte and Schubert,

4 Radiogenic heat production Radiogenic heat production, A or H, results from the decay of radioactive elements in the Earth, mainly 238 U, 235 U, 232 Th and K. A is generally used for volumetric heat production and H for heat production by mass. These elements occur in the mantle, but are concentrated in the crust, where radiogenic heating can be significant The surface heat flow in continental regions is ~65 mw m -2 and ~37 mw m -2 is from radiogenic heat production (57%) Concentration Rock Type U (ppm) Th (ppm) K (%) Reference undepleted (fertile) mantle Depleted peridotites Tholeiitic basalt Granite Shale Average continental crust Chondritic meteorites Turcotte and Schubert,

5 Radiogenic heat production Rock type By mass, H [W kg -1 ] Heat production By volume, A [W m -3 ] By volume, A [µw m -3 ] Reference undepleated (fer/le) mantle 7.39E E "Depleated" perido/tes 3.08E E Tholeii/c basalt 1.49E E Granite 1.14E E Shale 7.74E E Average con/nental crust 3.37E E Chondri/c meteorites 3.50E E Calculated from Turcotte and Schubert,

6 Radiogenic heat production Rock type By mass, H [W kg -1 ] Heat production By volume, A [W m -3 ] By volume, A [µw m -3 ] Reference undepleated (fer/le) mantle 7.39E E "Depleated" perido/tes 3.08E E Tholeii/c basalt 1.49E E Granite 1.14E E Shale 7.74E E Average con/nental crust 3.37E E Chondri/c meteorites 3.50E E Calculated from Turcotte and Schubert, 2014 Typical heat production values for upper crustal rocks are 2-3 µw m -3, but the crustal average is <1 µw m -3 What does this suggest, and why might this occur? 6

7 Radiogenic heat production Stüwe, 2007 H Crustal heat production over time Because radiogenic heat production is the result of radioactive decay, the parent isotope concentrations have continually decreased throughout Earth s history Today, the total amount of radiogenic heat is about half of what was produced in the Archean at ~3 Ga 7

8 Radiogenic heat production Stüwe, 2007 H Crustal heat production over time Because radiogenic heat production is the result of radioactive decay, the parent isotope concentrations have continually decreased throughout Earth s history Today, the total amount of radiogenic heat is about half of what was produced in the Archean at ~3 Ga 8

9 LETTER doi: /nature13728 Spreading continents kick-started plate tectonics Patrice F. Rey 1, Nicolas Coltice 2,3 & Nicolas Flament 1 A a b c d e Continent Temperature (K) Stresses acting on cold, thick and negatively buoyant oceanic 0 1,000 lithosphere are thought to be crucial to the initiation of subduction 0 and tectonic impact of a thick and buoyant continent surrounded by a stag- 2,000 that of present-day tectonic forces driving orogenesis 1. To explore the the operation of plate tectonics 1,2, which characterizes the presentday geodynamics of the Earth. Because the Earth s interior 200 was 1,820 K nant lithospheric lid, we produced a series of two-dimensional thermomechanical numerical models of the top 700 km of the Earth, using 293 hotter K in the Archaean eon, the oceanic crust may have been thicker, thereby temperature-dependent densities and visco-plastic rheologies that depend making the oceanic lithosphere more buoyant than at 0 present 3, and on temperature, melt fraction and depletion, stress and strain rate (see 0 km whether subduction and plate tectonics occurred during this time is Methods). The initial temperature field is the horizontally averaged temperature profile of a stagnant-lid convection calculation for a mantle 600 ambiguous, both in the geological record and in geodynamic models 4. Here we show that because the oceanic crust was thick and buoyant 5,,200 K hotter than at present (Fig. 1A, a and Extended Data Fig. 2). The early continents may have produced intra-lithospheric gravitational absence ofba lateraldeviatoric temperature gradients stress (MPa) ensures that Reference no convective density stresses(g cm 3 ) stresses large enough to drive their gravitational spreading, to initiate subduction at their margins and to trigger episodes of subduc- A buoyant and stiff 0 continent 225 km thick (strongly 0 depleted mantle act on the lid, allowing100 us to isolate 300 the 500 dynamic effects2.8 of the3.0 continent Basaltic crust tion. Our model predicts the co-occurrence of deep to progressively root 170 km thick overlain by felsic crust km thick; see Fig. 1B, a) is shallower mafic volcanics and arc magmatism within continents in inserted within the lid, on the left side of the domain to exploit Crust the symmetry of the problem (Fig. 1A, a). A mafic crust 15 km thick covers the a self-consistent geodynamic framework, explaining the enigmatic multimodal volcanism and tectonic record of Archaean cratons 6.Moreover, our model predicts a petrological stratification and tectonic struc- thick greenstone80 covers on continents, as well as80 thick basaltic Strongly crust on whole system (Fig. 1A, a), consistent with the common occurrence of ture of the sub-continental lithospheric mantle, two predictions that the oceanic lid 3. depleted are consistent with xenolith 5 and seismic studies, respectively, and consistent with the existence of a mid-lithospheric seismic discontinuity 7. tinent imparts 120 a horizontal force large enough120 to induce mantle a long period Our numerical solutions show that the presence oflithospheric a buoyant con- Depletion The slow gravitational collapse of early continents could have kickstarted transient episodes of plate tectonics until, as the Earth s inte- (, Myr) of slow collapse of the whole continental lithosphere Strain rate (Fig. 1 and Extended 160 Data Fig. rior cooled and oceanic lithosphere became heavier, plate tectonics ), s 1 in agreement with the dynamics of spreading for gravity currents 19.Hence,acontinentoflargervolumeleads 225 became self-sustaining. to larger gravitational power and faster collapse. BecauseAsthenosphere of lateral spreading of the continent, b the100 adjacent 300lithospheric 500 lid is slowly 2.8pushed 3.0 under Present-day plate tectonics is primarily driven by the negative buoyancy of cold subducting plates. Petrological and geochemical proxies of its margin (Fig. 1A, 0 b and Extended Data Fig. 3A, 0a). For gravitational Basaltic crust subduction preserved in early continents point to subduction-like processes already operating before 3 billion years (Gyr) ago 8,9 and perhaps of the lid is slow, and viscous drips (that is, Rayleigh Taylor instabilities) stress lower than the yield stress of the oceanic lid, thickening of the margin as early as 4.1 Gyr ago 10. However, they are not unequivocal, and geodynamic modelling suggests that the thicker basaltic crust produced by typical of stagnant-lid convection 20, mitigate the thermal thickening mantleof partial melting of a hotter Archaean or Hadean mantle would have had the lid detach from its base (Extended Data Fig. 3A, a and b). These Lithospheric instabilities, increased lithospheric buoyancy and inhibited subduction 3,4.Mantleconvection under a stagnant lid with extensive volcanism could therefore lid, subduction is initiated (Fig. 1A, b and c). Depending on the half-width When gravitational stresses overcome the yield stress of the lithospheric have preceded the onset of subduction 11. In this scenario, it is classically of the continent 120 and its density contrast with the120 adjacent oceanic lid (that assumed that the transition from stagnant-lid regime to mobile-lid regime is, its gravitational power) Strain three rate situations s can arise: first, Asthenosphere subduction and the onset of plate tectonics require that convective stresses overcame initiates and stalls (Extended Data Fig. 3b); second, 160the slab detaches and the strength of the stagnant lid 12 at some stage in the Archaean. the lid stabilizes (Fig. 1A, d and e and Extended Data Fig. 3c); or third, On the modern Earth, gravitational stresses due to continental buoyancy recurrent detachment of the slab continues until recycling of the oceanic can contribute to the initiation of subduction 2,13. The role of continental lid is completed, followed by stabilization (Extended Data Fig. 3d). When 9

10 LETTER doi: /nature13728 Spreading continents kick-started plate tectonics Patrice F. Rey 1, Nicolas Coltice 2,3 & Nicolas Flament 1 A a Continent Temperature (K) 0 1,000 2, km K 1,820 K b Continent weakens, spreads laterally Ba Deviatoric stress (MPa) Reference density (g cm 3 ) Basaltic crust Crust c d Depletion Larger horizontal stresses force subduction of buoyant oceanic lithosphere b Strain rate s Strongly depleted lithospheric mantle Asthenosphere Basaltic crust e Slab detaches, margin stabilizes Strain rate s Lithospheric mantle Asthenosphere 10

11 Let s see what you ve learned If you re watching this lecture in Moodle, you will now be automatically directed to the quiz! References: Rey, P. F., Coltice, N., & Flament, N. (2015). Spreading continents kick-started plate tectonics. Nature, 513(7518), 5 8. doi: /nature